The present invention relates to a delay detection circuit used for PSK (Phase Shift Keying) and FSK (Frequency Shift Keying).
Conventionally, a carrier regenerative system using a narrow-band filter, for example, that detects a phase difference from a demodulation output by synchronous detection, controls a phase of a regenerative carrier based on the phase difference, and compensates the phase difference of the regenerative carrier caused by a frequency deviation is known as shown in Japanese Patent Laid-Open No. 61-117957(1986).
However, this system has a disadvantage in the field where its receiver must be miniaturized by integration, especially in the field of mobile communications, because the system needs a phase shifter.
On the other hand, comparing this synchronous detection system, a latest delay detection system that demodulates a digital-phase-modulated carrier is considered to be fit to use for mobile communications because burst frame efficiency and error rate characteristic under Rayleigh fading (where probability distribution of a received power is approximated to a Rayleigh distribution when many multiple waves interfere with each other at random) thereof are better than that.
The delay detection system, however, has a defect that its deterioration of error rate tends to be remarkable if a frequency shift occurs by transmission frequency drift, etc. The reason of this phenomenon is that a receiver detects the frequency shift as a phase rotation of normal reception phase difference.
For solving this problem, a delay detection circuit to compensate the frequency shift is proposed. The delay detection circuit shown in FIG. 3, for example, detects, in a modulator 31, an ideal phase difference 303 of a demodulation symbol 302 estimated in a demodulator 30. Then, it estimates a phase compensation quantity 304 by calculating a moving average of a difference between a reception phase difference 301 and the ideal phase difference 303 using a phase compensation quantity estimator 32, obtains a compensation phase difference 305 by compensating a frequency offset of a frequency shift based on the phase compensation quantity 304 using an adder 33, feedbacks this compensation phase difference 305 to the demodulator 30.
However, such a delay detection circuit has some problems, such as a reception signal performed by frequency offset is apt to be regarded as a noise and an instantaneous frequency offset quantity is misdetected because a reception symbol is not detected correctly if a phase rotation by frequency offset goes over 45.degree.. As a result, constant and correct compensation of reception signal can not be performed.
To solve these problems, a delay detection circuit to compensate frequency offset is also proposed. This delay detection circuit, shown in FIG. 4 as an example, detects a specific pattern from demodulation symbols 401 using a detector 40, simultaneously outputs an ideal phase difference 402 of the specific pattern. Besides this, it estimates a phase compensation quantity 404 in a phase compensation quantity estimator 41 by averaging differences of the ideal phase difference 402 and a reception phase difference 403 delayed in a specific number of patterns in a delay circuit 42, compensates a frequency offset to obtain a compensation phase difference 405 in an adder 43 based on the phase compensation quantity 404, feedbacks the compensation phase difference 405 to a demodulator 44.
Generally, when a receiver asynchronously receives a burst signal, an already-known symbol such as a preamble is sent in a specific pattern for a relatively long time interval earlier than a data signal that is sent as information. On the other hand, if it becomes synchronous status, the transmission time interval becomes relatively short for increasing burst frame efficiency.
Additionally, when a burst signal is received using the delay detection circuit shown in FIG. 4, there is a defect that a frequency offset can not be compensated if the already-known specific pattern can not be recognized. For example, if it is needed to asynchronously receive a burst signal in a condition that error rate is high, the error rate may be deteriorated further by frequency offset. In such a case, compensation may need longer time, since the initial specific pattern is difficult to be detected.
Also, if the transmission time of the specific pattern even in a synchronous status, this delay detection circuit may not detect a correct compensation quantity because it can not fully average an instantaneous phase compensation quantity to be detected. This means that a conventional delay detection circuit can not rapidly and fully perform phase compensation. More, there is a problem that the reception error rate of this delay detection circuit can not be improved fully when frequency offset is performed.